On the bais of ab initio calculations of double-zeta quality at a SCF and MCSCF level, bonding in bis-ylenephosphoranes is compared with the one in phosphoranes. The investigations include the series H3PX, HPX2, HPX (X = CH2, NH, O, SiH2, PH, S). The bis-ylenephosphorane formation becomes more favorable with increasing electronegativity of X. An extreme case is the electropositive group X = SiH2. The resulting pi-system suffers from the Jahn-Teller distortion, causing reduction from C2v to lower C2 Symmetry in the corresponding bisylenephosphorane. Energetically, the corresponding ring systems are more stable than their bis-ylenephoshorane counterparts (for X = CH2 and PH). The electronic hypersurface of the hitherto unknown bis(phosphinylidene)phosphorane is explored in detail and compared in regard to bonding with its analog bis(methylene)phosphorane. For both cases pi-push-pull substitution is important for stabilization of the planar allylic geometry.